intel/fs: optimize uniform SSBO & shared loads

Using divergence analysis, figure out when SSBO & shared memory loads
are uniform and carry the data only once in register space.

Signed-off-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/21853>

src/intel/compiler/brw_fs_nir.cpp

@@ -4938,6 +4938,62 @@ fs_visitor::nir_emit_intrinsic(const fs_builder &bld, nir_intrinsic_instr *instr
       break;
    }
 
+   case nir_intrinsic_load_ssbo_uniform_block_intel:
+   case nir_intrinsic_load_shared_uniform_block_intel: {
+      fs_reg srcs[SURFACE_LOGICAL_NUM_SRCS];
+
+      const bool is_ssbo =
+         instr->intrinsic == nir_intrinsic_load_ssbo_uniform_block_intel;
+      srcs[SURFACE_LOGICAL_SRC_SURFACE] = is_ssbo ?
+         get_nir_ssbo_intrinsic_index(bld, instr) : fs_reg(brw_imm_ud(GFX7_BTI_SLM));
+
+      const unsigned total_dwords = ALIGN(instr->num_components, REG_SIZE / 4);
+      unsigned loaded_dwords = 0;
+
+      const fs_builder ubld1 = bld.exec_all().group(1, 0);
+      const fs_builder ubld8 = bld.exec_all().group(8, 0);
+      const fs_builder ubld16 = bld.exec_all().group(16, 0);
+
+      const fs_reg packed_consts =
+         ubld1.vgrf(BRW_REGISTER_TYPE_UD, total_dwords);
+
+      const nir_src load_offset = is_ssbo ? instr->src[1] : instr->src[0];
+      if (nir_src_is_const(load_offset)) {
+         fs_reg addr = ubld8.vgrf(BRW_REGISTER_TYPE_UD);
+         ubld8.MOV(addr, brw_imm_ud(nir_src_as_uint(load_offset)));
+         srcs[SURFACE_LOGICAL_SRC_ADDRESS] = component(addr, 0);
+      } else {
+         srcs[SURFACE_LOGICAL_SRC_ADDRESS] =
+            bld.emit_uniformize(get_nir_src(load_offset));
+      }
+
+      while (loaded_dwords < total_dwords) {
+         const unsigned block =
+            choose_oword_block_size_dwords(devinfo,
+                                           total_dwords - loaded_dwords);
+         const unsigned block_bytes = block * 4;
+
+         srcs[SURFACE_LOGICAL_SRC_IMM_ARG] = brw_imm_ud(block);
+
+         const fs_builder &ubld = block <= 8 ? ubld8 : ubld16;
+         ubld.emit(SHADER_OPCODE_UNALIGNED_OWORD_BLOCK_READ_LOGICAL,
+                   retype(byte_offset(packed_consts, loaded_dwords * 4), BRW_REGISTER_TYPE_UD),
+                   srcs, SURFACE_LOGICAL_NUM_SRCS)->size_written = align(block_bytes, REG_SIZE);
+
+         loaded_dwords += block;
+
+         ubld1.ADD(srcs[SURFACE_LOGICAL_SRC_ADDRESS],
+                   srcs[SURFACE_LOGICAL_SRC_ADDRESS],
+                   brw_imm_ud(block_bytes));
+      }
+
+      for (unsigned c = 0; c < instr->num_components; c++)
+         bld.MOV(retype(offset(dest, bld, c), BRW_REGISTER_TYPE_UD),
+                 component(packed_consts, c));
+
+      break;
+   }
+
    case nir_intrinsic_store_output: {
       assert(nir_src_bit_size(instr->src[0]) == 32);
       fs_reg src = get_nir_src(instr->src[0]);

src/intel/compiler/brw_nir.c

@@ -1301,11 +1301,25 @@ brw_nir_should_vectorize_mem(unsigned align_mul, unsigned align_offset,
    if (bit_size > 32)
       return false;
 
-   /* We can handle at most a vec4 right now.  Anything bigger would get
-    * immediately split by brw_nir_lower_mem_access_bit_sizes anyway.
-    */
-   if (num_components > 4)
-      return false;
+   if (low->intrinsic == nir_intrinsic_load_ssbo_uniform_block_intel ||
+       low->intrinsic == nir_intrinsic_load_shared_uniform_block_intel) {
+      if (num_components > 4) {
+         if (!util_is_power_of_two_nonzero(num_components))
+            return false;
+
+         if (bit_size != 32)
+            return false;
+
+         if (num_components > 32)
+            return false;
+      }
+   } else {
+      /* We can handle at most a vec4 right now.  Anything bigger would get
+       * immediately split by brw_nir_lower_mem_access_bit_sizes anyway.
+       */
+      if (num_components > 4)
+         return false;
+   }
 
 
    uint32_t align;
@@ -1447,6 +1461,31 @@ brw_vectorize_lower_mem_access(nir_shader *nir,
       }
 
       OPT(nir_opt_load_store_vectorize, &options);
+
+      /* Only run the blockify optimization on Gfx9+ because although prior HW
+       * versions have support for block loads, they do have limitations on
+       * alignment as well as requiring split sends which are not supported
+       * there.
+       */
+      if (compiler->devinfo->ver >= 9) {
+         /* Required for nir_divergence_analysis() */
+         OPT(nir_convert_to_lcssa, true, true);
+
+         /* When HW supports block loads, using the divergence analysis, try
+          * to find uniform SSBO loads and turn them into block loads.
+          *
+          * Rerun the vectorizer after that to make the largest possible block
+          * loads.
+          *
+          * This is a win on 2 fronts :
+          *   - fewer send messages
+          *   - reduced register pressure
+          */
+         nir_divergence_analysis(nir);
+         if (OPT(brw_nir_blockify_uniform_loads, compiler->devinfo))
+            OPT(nir_opt_load_store_vectorize, &options);
+         OPT(nir_opt_remove_phis);
+      }
    }
 
    OPT(nir_lower_mem_access_bit_sizes,

src/intel/compiler/brw_nir.h

@@ -187,6 +187,9 @@ bool brw_nir_opt_peephole_imul32x16(nir_shader *shader);
 bool brw_nir_clamp_per_vertex_loads(nir_shader *shader,
                                     unsigned input_vertices);
 
+bool brw_nir_blockify_uniform_loads(nir_shader *shader,
+                                    const struct intel_device_info *devinfo);
+
 void brw_nir_optimize(nir_shader *nir,
                       const struct brw_compiler *compiler,
                       bool is_scalar);

src/intel/compiler/brw_nir_blockify_uniform_loads.c

@@ -0,0 +1,101 @@
+/*
+ * Copyright © 2018 Intel Corporation
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a
+ * copy of this software and associated documentation files (the "Software"),
+ * to deal in the Software without restriction, including without limitation
+ * the rights to use, copy, modify, merge, publish, distribute, sublicense,
+ * and/or sell copies of the Software, and to permit persons to whom the
+ * Software is furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice (including the next
+ * paragraph) shall be included in all copies or substantial portions of the
+ * Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.  IN NO EVENT SHALL
+ * THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
+ * FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS
+ * IN THE SOFTWARE.
+ */
+
+#include "isl/isl.h"
+
+#include "brw_nir.h"
+
+static bool
+brw_nir_blockify_uniform_loads_instr(nir_builder *b,
+                                     nir_instr *instr,
+                                     void *cb_data)
+{
+   if (instr->type != nir_instr_type_intrinsic)
+      return false;
+
+   const struct intel_device_info *devinfo = cb_data;
+
+   nir_intrinsic_instr *intrin = nir_instr_as_intrinsic(instr);
+   switch (intrin->intrinsic) {
+   case nir_intrinsic_load_ssbo:
+      /* BDW PRMs, Volume 7: 3D-Media-GPGPU: OWord Block ReadWrite:
+       *
+       *    "The surface base address must be OWord-aligned."
+       *
+       * We can't make that guarantee with SSBOs where the alignment is
+       * 4bytes.
+       */
+      if (devinfo->ver < 9)
+         return false;
+
+      if (nir_src_is_divergent(intrin->src[1]))
+         return false;
+
+      if (nir_dest_bit_size(intrin->dest) != 32)
+         return false;
+
+      /* Without the LSC, we can only do block loads of at least 4dwords (1
+       * oword).
+       */
+      if (!devinfo->has_lsc && nir_dest_num_components(intrin->dest) < 4)
+         return false;
+
+      intrin->intrinsic = nir_intrinsic_load_ssbo_uniform_block_intel;
+      return true;
+
+   case nir_intrinsic_load_shared:
+      /* Block loads on shared memory are not supported before the LSC. */
+      if (!devinfo->has_lsc)
+         return false;
+
+      if (nir_src_is_divergent(intrin->src[0]))
+         return false;
+
+      if (nir_dest_bit_size(intrin->dest) != 32)
+         return false;
+
+      /* Without the LSC, we can only do block loads of at least 4dwords (1
+       * oword).
+       */
+      if (!devinfo->has_lsc && nir_dest_num_components(intrin->dest) < 4)
+         return false;
+
+      intrin->intrinsic = nir_intrinsic_load_shared_uniform_block_intel;
+      return true;
+
+   default:
+      return false;
+   }
+}
+
+bool
+brw_nir_blockify_uniform_loads(nir_shader *shader,
+                               const struct intel_device_info *devinfo)
+{
+   return nir_shader_instructions_pass(shader,
+                                       brw_nir_blockify_uniform_loads_instr,
+                                       nir_metadata_block_index |
+                                       nir_metadata_dominance |
+                                       nir_metadata_live_ssa_defs,
+                                       (void *) devinfo);
+}

src/intel/compiler/meson.build

@@ -86,6 +86,7 @@ libintel_compiler_files = files(
   'brw_nir_analyze_boolean_resolves.c',
   'brw_nir_analyze_ubo_ranges.c',
   'brw_nir_attribute_workarounds.c',
+  'brw_nir_blockify_uniform_loads.c',
   'brw_nir_clamp_per_vertex_loads.c',
   'brw_nir_lower_conversions.c',
   'brw_nir_lower_cs_intrinsics.c',